protosw: this is a kernel only header, put back #ifdef _KERNEL

The uncommented #ifdef _KERNEL originates from 1997 by wollman@ (see
57bf258e3dc9e) with no clear original reason. Since these old times two
abusers of the header leak were created - libprocstat(3) and netstat(1).
These two already have a lot of _WANT_FOO hacks, so satisfy them with
_WANT_PROTOSW.

While here, cleanup and sort forward declaraions.